The invention relates to a circuit arrangement for generating a control signal, and to a radio receiver including such a circuit arrangement.
EP-A 0 160 390, which corresponds to U.S. Pat. No. 4,672,636, discloses a circuit arrangement for an FM receiver which is arranged to receive directly modulated data signals. This circuit arrangement includes a local oscillator whose frequency lies on the frequency scale between two signal frequencies. The known circuit arrangement is arranged notably for the reception and demodulation of FSK modulated signals as used in digital radio receivers (pagers).
The circuit arrangement known from EP-A 0 160 339 includes a mixer in which signals originating from an antenna are mixed with the signal from a local oscillator. A channel filter which succeeds the mixer and is constructed as a low-pass or bandpass filter filters the output signal of the mixer and applies it to a stage whose transfer function has the characteristic of a frequency discriminator. The output signal thereof is applied, via an amplifier and a low-pass filter, as a control signal to the local oscillator in order to readjust the frequency thereof. The article "Zwei ICs fur einen Pager" by Stephan Drude, Funkschau, Heft 26, 1989, pp. 69 to 76, describes a receiver component, having the type number UAA2050T, for a pager operating with direct frequency shift keying, i.e. with FSK modulation. A pager constructed by means of such a receiver component includes automatic frequency control (AFC) for the compensation of temperature fluctuations and ageing effects.
DE-A-29 42 512 describes a radio receiver for receiving FSK modulated radio signals via an antenna. The radio signals contain two receiving frequencies which are situated a distance equal to the frequency deviation of the FSK modulation above and below the frequency of an RF carrier. These radio signals are applied to two high-gain mixers. A mixing oscillator oscillates at the frequency of the RF carrier. Its signal is applied to the first high-gain mixer directly and to the second high-gain mixer via a 90.degree. phase shifter. The outputs of the mixers are connected to a respective low-pass filter. The filtered signals then reach a respective high-gain limiter amplifier. The outputs of the limiter amplifiers supply square-wave signals. The signal at the output of one of the limiter amplifiers leads or lags the signal at the output of the other limiter amplifier, depending on whether the frequency of the input signal at the antenna is lower or higher than that of the mixer oscillator. These two feasible states are recognized by a D-flipflop which is switched to one of its two feasible states in dependence thereon.
The frequency of an ideal receiving signal for such a radio receiver, for example a pager, changes abruptly between the two feasible values, i.e. the two feasible frequencies of the radio signals received which are also referred to as transmitter signal "1" or "0" in the radio signal data code. In reality, however, such an ideal signal is not present; to the contrary, the transmitter signal requires a finite period of time for changing over from one frequency to the other. Analogously, a finite period of time elapses during the switching over from one state to the other in the demodulated signal. It appears that valid frequency measurement is not possible during this period of time. During this period of time, referred to as a transitional interval, not only the assignment of the transmitted radio signals to one of the data values "1" or "0" is impeded, but notably also the generating of the control signal for the controllable oscillator. This is because this control signal must correspond to the frequency deviation of the FSK modulation in order to achieve correct control of the oscillator frequency. During the transitional interval, i.e. during the period of time in which the data signal derived from the received signal changes its value, however, a control signal is generated which no longer corresponds to the frequency deviation of the FSK modulation, but notably to a smaller frequency difference. This results in undue readjustment of the frequency of the oscillator during the transitional intervals.
The associated error occurs notably in the case of high information transmission rates in the receiving signal with the resultant very short information units (bits). This is because the transitional intervals then take up a substantial part of the overall duration of the individual bits. This increases the risk of the oscillator frequency control being disturbed by the generating of an erroneous control signal. Errors then occur increasingly in the data reception of the radio receiver; these errors must be avoided so as to ensure that a required low bit error rate of the radio receiver (for example, a pager) is not exceeded.
Because of the receiving frequencies of contemporary radio receivers (pagers), amounting to, for example approximately 930 MHz, very severe requirements are also imposed in respect of the short-term and the long-term stability of the frequency of the oscillator of the receiving device. If the oscillator cannot satisfy these requirements, the sensitivity will inevitably be affected and in some cases reception will even break down.